1. Field of the Invention
The invention is directed to cellulose derivatives that have a low degree of substitution and a uniform degree of modification along the polymer backbone. The invention is particularly directed to an acylation process for producing the cellulose derivatives.
2. Description of the Prior Art
The chemical modification of cellulose, with esterifying and etherifying agents, represents a widely practiced industrial technology worldwide. Both cellulose ethers and cellulose esters have reached significant commercial importance. All commercially available cellulose derivatives are made by heterogenous reaction. Malm et al., Ind. Eng. Chem., 43(3), 688-91 (1951) discloses two phase reaction chemistry in which cellulose remains suspended in a solution of a reagent in water or a solvent. The cellulose reacts gradually in a stepwise fashion beginning with the amorphous regions and proceeding to the crystalline regions. Uryash et al., Thermochim. Acta, 93:409-412 (1985), reported that cellulosic materials made using the two-stage reaction chemistry have a "blocky" character where neighboring sections can be unsubstituted or have a high degree of substitution or modification, depending on the accessibility.
The practice of heterogenous reaction chemistry in connection with cellulose esters has resulted in products with a high degree of substitution, usually above 2.4. This means that on average, 2.4 of the hydroxy groups per sugar molecule in the cellulose backbone are esterified. Highly esterified cellulose materials have good solubility, improved thermal properties, and improved processability. However, cellulose esters with a high degree of substitution have reduced biodegradability, decreased moisture uptake, and decreased interaction with polar substances. In addition, commercially available cellulose derivatives produced using the two-phase reaction chemistry are limited to ester derivatives with acyl substituents having less than four carbon atoms (C.sub.4).
McCormick & Callais, Polymer, 28:2317-23 (1987) have proposed the acylation of cellulose in lithium chloride (LiCl)/N,N-dimethyl acetamide (DMAC) using acyl chlorides and acid anhydrides of small size (less than C.sub.3). While acyl chlorides are highly reactive, they are collectively insoluble in the LiCl/DMAc solvent system, except for acetyl chloride. Homogenous phase reaction conditions are lost in reactions with propionyl or higher acyl chlorides.
Shimizu and Hayashi, Cellulose Chemistry and Technology 23, 667-670 (1989) reported cellulose esterification using p-toluenesulphonyl chloride (TsCl). Specifically, Shimizu et al. disclosed the production of tri-substituted cellulose esters from an acetylation reaction.